1. Field of the Invention
The present invention relates generally to aircraft control systems and more particularly to aircraft performance management systems for controlling the aircraft vertical flight profile through the coordinated operation of the engine throttles and pitch attitude to thereby provide the most cost effective operation of the aircraft.
2. Description of the Prior Art
Most modern commercial transport aircraft, many general aviation aircraft and certain military aircraft include automatic flight control systems and/or flight director systems, for controlling the aircraft attitude and flight path as well as automatic throttle control systems for controlling aircraft thrust. For many years these two systems have been operated more or less independently or coordinated only during certain phases of flight, such as for example during take-off and landing operations, the primary consideration being aircraft controllability and safety rather than operating costs. However, in view of the recent substantial increases in fuel costs, aircraft operators are very desirous of increasing fuel efficiencies throughout the entire vertical flight profile of their flight plans by assuring the most cost effective operations possible. Coordination between the automatic throttle controls and the automatic pitch attitude flight controls have been carefully analyzed in attempts to achieve these results. Recently developed systems, resulting in this coordinated automatic thrust/attitude control for aircraft have been termed flight management or performance management systems. In general, such systems compute and control thrust and pitch parameters to achieve minimum fuel consumption within the constraints of scheduled trip time between the start of the take-off run to touchdown at the destination. A particular objective of such systems is to obtain smooth, stable and accurate control during cruise and when the automatic system commands the aircraft to accelerate, decelerate or change its flight path in the vertical plane.
At normal high altitude cruise speeds selected or computed for optimum fuel economy, modern, low drag aircraft tend to have an almost neutral speed stability; that is, small changes in engine thrust or engine pressure ratio (EPR) produce large changes in Mach number speed, or conversely, Mach speed is very sensitive to small changes in the engine throttle. Manual control of EPR to maintain the fuel efficient Mach speed is difficult since it requires constant pilot attention. Commanding a higher or lower Mach speed at which the aircraft is more speed stable is inefficient in terms of fuel consumption. In some prior art speed control systems, the error between commanded Mach and actual Mach was used to control EPR. This type of system is not fully satisfactory and produced speed hunting because of the above inherent neutral speed stability. If the system gains were lowered to reduce hunting, the control was overly sluggish and/or inaccurate. One attempt in the prior art to overcome the neutral speed stability problem at cruise speeds is disclosed in U.S. Pat. No. 4,277,041. In this system Mach error, Mach integral and the rate of change of actual Mach are used to control the autothrottle or EPR but in addition, actual Mach rate or washed out actual Mach is supplied to the autopilot pitch channel causing a temporary altitude error. This short term Mach to autopilot pitch allows short term variations in speed to be controlled by changes in altitude (pitch) rather than by throttle activity which is reflected in throttle movement and is distracting to the flight crew and wasteful of fuel. One of the difficulties with this prior cruise control configuration is that the basic airspeed terms, Mach error, Mach rate or pseudo rate, and Mach error integral are still used directly in the throttle control loop, which can still result in excessive throttle activity and excessive fuel consumption. If the integral term only is used, as in turbulence, loss of precise speed control will result.
It is also recognized that there have been other aircraft control systems involving the cross feed of throttle control signals to the pitch autopilot. One of these is Applicant's Assignee's U.S. Pat. No. 2,933,268 issued to Jude et al. In this patent airspeed error and error rate are used to position the aircraft throttles so as to maintain a selected airspeed. At the same time a throttle position signal is cross fed to the pitch channel of the autopilot, operable in a glide slope hold or altitude hold mode to anticipate and prevent any flight path change due to throttle changes. In the cruise speed control described in this patent, the throttle control system has the dual function of controlling throttles and vernier changes in altitude while the autopilot pitch channel has the dual function of controlling altitude and vernier changes in speed. The Jude et al system does not address the speed stability problem associated with fuel efficient cruise control and will not solve the excessive throttle activity problem solved by the present invention.